This application is related to Japanese applications Nos. Hei 11 (1999)-332277 and Hei 11 (1999)-332276, both filed on Nov. 24, 1999 whose priorities are claimed under 35 USC xc2xa7119, the disclosures of which are incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to a coating liquid for forming an organic electroluminescence (EL) element which allows full-color display, the organic EL element and a manufacture method of the same.
2. Description of Related Art
According to a highly advanced technology in recent years, display devices reduced in size, power consumption and weight have been demanded and attention has been paid to high luminance and low-voltage-drive organic electroluminescence (EL) display devices. As a result of recent research and development, organic EL elements made of organic materials exhibit marked improvement in light emitting efficiency and practical application thereof has been started. In view of manufacture of such organic EL display devices, polymer materials have been used in expectation of reduced production costs since they can be formed into films by coating. Accordingly, a method for patterning such polymer materials into light emitting layers in order to obtain organic EL color display devices has currently been researched.
There is a close relationship between a film forming process and a patterning process. A dry process and a wet process have been employed for the film formation, among which the wet process takes a merit of easier film formation. Examples of the patterning method used in the wet process include, for example, electrodeposition (Japanese Unexamined Patent Publication No. Hei 9 (1997)-7768), ink jet technique (Japanese Unexamined Patent Publication No. Hei 10 (1998)-12377), and printing method (Japanese Unexamined Patent Publications Nos. Hei 3 (1991)- 269995, Hei 10 (1998)-77467 and Hei 11 (1999)-273859).
However, in the electrodeposition, a film of favorable quality cannot be formed and usable materials are limited. The ink jet technique cannot form a film of favorable surface configuration. A film of favorable quality and surface configuration can be formed by the printing method.
In the case where a thin film of 1 xcexcm or less as required for the organic EL element is aimed, it is necessary to apply the film much thinner in a wet state than in a normal printing.
However, where solvents such as chloroform, methylene chloride, dichloroethane, tetrahydrofuran, toluene and xylene described in Examples of the above-mentioned Japanese Unexamined Patent Publication No. Hei 3 (1991)-269995 are used in a coating liquid and applied thin in the wet state, the coating liquid is dried on a roller or a transfer substrate. Accordingly, it is impossible to form a desired film on a substrate by the printing method. Further, the above-mentioned Japanese Unexamined Patent Publications Nos. Hei 10 (1998)-77467 and Hei 11 (1999)-273859 use a micro gravure coater to obtain the thin film of 1 xcexcm or less, in which the roller for applying the coating liquid to the substrate rotates in the direction opposite to that of the substrate movement. As a result, a patterned film cannot be obtained and color display by light emitting layers of different colors cannot be realized. Even if the thin film is obtained with the coating liquid of reduced concentration, a metal mesh used for screen printing leaves its mark, which may cause short circuit or irregular light emission.
In the case of forming a full color light emitting element with the different colored light emitting layers by the printing method as described above, it is preferred to provide ribs between pixels of the light emitting layers to prevent mixing of the light emitting layers.
However, where the coating liquid is applied by the printing method onto the substrate provided with the ribs, the coating liquid is not applied to the neighborhood of the ribs, that is, portions without an organic electroluminescence (EL) layer are resulted. Where electrodes are formed on the organic EL layers and voltage is applied, short circuit between the electrodes occurs at the portion without the organic EL layer. Therefore the element does not emit the light. FIGS. 16(a) and 16(b) show the portion in which the organic EL layer is not formed due to the existence of the ribs. Reference numeral 5 denotes the rib, 3 the organic EL layer and 22 the portion without the organic EL layer. Where the ribs are arranged in stripes as shown in FIG. 16(a), the portions 22 without the organic EL layers are formed in stripes along the ribs. Where the ribs are formed into the enclosure shape as shown in FIG. 16(b), the portions without the organic EL layers are generated in the enclosure shape along the inside of the ribs.
If narrow electrodes are formed so as not to overlap with the portions to which the coating liquid is not applied to prevent the short circuit, an area shared by the light emitting region in the organic EL element is reduced. FIG. 17 shows an example where a narrow second electrode 4 is formed on the organic EL layer 3. When the second electrode 4 is configured so as not to overlap with the portions without the organic EL layers, an intersectional portion of a first electrode 2 and the second electrode 4 serves as a light emitting region 23. Thus, the light emitting region becomes smaller and a percentage of area shared by the light emitting region in the organic EL element is reduced.
The present invention has been achieved in view of the above drawbacks. The inventor of the invention has found that the drying of the coating liquid during the printing step is closely related to the vapor pressure of the solvent used and that the drying can be avoided by adding at least one solvent of low vapor pressure.
Further, the inventor has found that the mixing of the light emitting layers formed by letterpress printing can be prevented by providing the ribs. He has also found that the generation of the portions to which the coating liquid is not applied is effectively avoided by combining the letterpress printing with spin coating, bar coating, dip coating, discharge coating or vapor deposition.
According to the present invention, provided is a coating liquid for forming an organic layer of an organic EL element by a printing method, comprising at least one organic solvent showing a vapor pressure of 500 Pa or less at a temperature for forming the organic layer.
The present invention further provides an organic EL element in which at least one organic layer comprising the organic EL layer is formed by using the coating liquid for forming the organic EL layer described above.
Still according to the present invention, provided is a method for forming an organic EL element comprising a substrate, an organic EL layer constructed of a single organic layer or plural organic layers and a pair of electrodes sandwiching the organic EL layer, wherein at least one organic layer of the organic EL layer is formed by a printing method utilizing the coating liquid for forming the organic EL layer described above and a printer.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes. and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.